1. Field of the Invention
The present invention relates generally to the field of optical devices for optical fiber communications systems. More specifically, the present invention discloses a traveling wave Mach-Zehnder modulator providing negative chirp.
2. Statement of the Problem
Mach-Zehnder modulators (MZMs) are commonly used as transmitters in optical fiber communications systems. One critical parameter in such systems is the amount of dynamic shift in the instantaneous frequency of the optical carrier as it is being modulated with data. Typical data symbol rates are in the range of 2.5 GBd to 100 GBd, while typical optical carrier frequencies are in the range of 190 THz to 210 THz. An induced shift in the instantaneous frequency of the carrier is commonly called frequency chirp, or simply chirp, and can be an amount up to 20 GHz or more, to lower or higher frequencies than the optical center frequency. In many modulated signals, particularly those produced by other means such as directly modulated lasers, and especially those employing a modulation format based on on-off keying (OOK) of the optical power, chirp is an unwanted side effect of the modulation process. Since optical fiber is dispersive, the frequency spectrum of the modulated signal is a dominating factor affecting the quality of the signal as it propagates down a fiber.
Among the well known advantages of MZMs is the ability to design it to provide chirp to meet the needs of a specific application. For example, zero chirp (ZC) MZMs, which produce substantially no chirp, are widely available from commercial sources. Other MZMs are designed specifically to produce a chirp which behaves in opposition to the fibre dispersion, thereby extending the length of fiber down which the signal can propagate before experiencing unacceptable degradation. For fibers with positive dispersion, these MZMs are known as negative chirp (NC) MZMs, and are also generally available from a variety of commercial sources. A novel and advantageous design of a Mach-Zehnder optical device capable of a chirped output optical signal, such as a NC MZM, is the subject of this invention.
The basic structure of a prior art lumped element InP/InGaAsP Y-junction MZM is shown in FIG. 1, as described for example by Yu in U.S. Pat. No. 5,991,471. It consists of an input optical waveguide 1, connected to a Y-junction coupler 2 which splits the optical signal into two paths 3a and 3b. In each of the two optical paths is a modulation electrode 4a and 4b for shifting the relative optical phase between the two signal paths with the data signals; pads 5 for electrical connection to external modulation signals; a Y-junction coupler 6 which combines the two paths into an output waveguide 7. As described by Yu, there may be an additional means to make direct current (DC) adjustments to the relative phase independent of the modulation signal. Furthermore, Yu teaches that modulation electrodes 4a and 4b may be of different lengths, which can be advantageous for producing an intentional chirp. However, Yu also points out that if modulation electrodes 4a and 4b do have different lengths, the bandwidth of the MZM will be affected in a disadvantageous manner. Yu proposes that the pads 5 can be adjusted to different sizes to provide a capacitive balance to the asymmetric electrodes 4a and 4b. The symbol rate of such lumped element MZM architectures is typically limited to about 10 GBd even without the disadvantageous effect of asymmetric electrodes.
Another method of controlling chirp is by separating the amplitude and phase modulation operations. A MZM may be used to modulate the power level. The optical input of the MZM is connected to the optical output of a phase modulator which is also driven by an electrical signal. As shown in FIG. 2, it is known that the modulation electrodes 8 and 9 of the MZM can be continued, with one of them placed over the incoming waveguide, such that the incoming waveguide under the electrode becomes a phase modulator. As taught in U.S. Pat. No. 5,408,544 (Seino), the two electrodes may be traveling wave electrodes (TWE), meaning that they also act as an electrical transmission line and support a radio frequency (RF) electrical mode.
The prior art of modulator disclosed by Seino and depicted in FIG. 2 has a number of disadvantages. If it is designed as a lumped element device then, like the prior art of FIG. 1 with unequal electrode lengths, the bandwidth will disadvantageously affected. If it is designed as a TWE device, a number of problems are present. The modulation efficiency and bandwidth is advantageously affected if the characteristic impedance of the transmission line formed by the modulation electrodes 8 and 9 is matched to the electrical source, typically 50 Ohms. Bandwidth is further advantageously affected if the propagation velocity of the RF mode is substantially the same as the propagation velocity of the optical mode contained in the waveguides. A fully-optimized device, therefore, would have both the impedance and velocities matched simultaneously. Both of these factors are controlled by the dimensions (e.g., gap width, 10) of the modulation electrodes. In devices which rely on fringing electric fields to supply a phase change to the optical signals, such as lithium niobate (LiNbO3), the gap width also determines the modulation efficiency, i.e., how much voltage or how long an electrode is required to provide sufficient phase change. In the prior art of Seino, there is no capability to simultaneously achieve all three optimization goals: impedance, velocity match, and modulation efficiency. Note that typically LiNbO3 modulators are roughly 1-4 cm in length, whereas MZMs made from alternative semiconductor materials, such as indium gallium arsenide phosphide on indium phosphide substrates (InGaAsP/InP), have chips lengths about one tenth that of LiNbO3, 1-4 mm. Materials such as InGaAsP/InP can be designed such that the electrical signal can be delivered efficiently to the optical waveguides without relying on fringing fields. However, doing so causes the modulation electrodes to have a high capacitance, which, in the configuration of FIG. 3 would disadvantageously affect both the bandwidth and modulation efficiency.
A further practical problem in implementing the prior art disclosed by Seino is the splitting means 11. The splitting means 11 is extremely sensitive to manufacturing tolerances and perturbations to the optical phase of the light beam passing through it. Seino does not teach how to avoid applying phase modulation to this sensitive element, thereby affecting the optical loss and/or splitting ratio. Routing the modulation electrode far enough away from the element so that it is unaffected by the electrical signal would cause a change in the RF properties of the electrodes.
An alternative prior art MZM is shown schematically in FIG. 3, and is described in detail by R. G. Walker, “High-speed III-V semiconductor intensity modulators”, IEEE J. Quantum Electron., Vol. 27(3), p. 654, 1991. This architecture is referred to herein as a loaded line traveling wave electrode (LLTWE) MZM. The arrangement is distinguished by the presence of two types of electrically-connected electrodes. First, there is a pair of transmission line electrodes 14 which support a radio frequency (RF) guided mode. Second, modulation phase shifting is provided by a plurality of modulation electrode pairs 15, with one electrode of each pair adjacent to each parallel MZM arm. The modulation electrode pairs are arranged periodically end-to-end along the length of the MZM arms. Having transmission line electrodes separate from modulation electrodes is extremely advantageous in that it allows the transmission line properties to be decoupled from the modulation electrode properties. The modulation electrodes now provide a periodic capacitive load to the transmission line electrodes. Using this architecture, it is possible to simultaneous match the TWE impedance to a modulation signal source, and to match the propagation velocities of the RF and optical guided modes, without a direct impact on the efficiency of the modulation electrodes. A further distinguishing feature compared to Yu of some MZMs of this type is the ability to operate in series push-pull configuration. As described in the Walker article, the TWE MZM can be configured such that pairs of modulation sections (one of each pair on each of the two MZM arms) are electrically in series, but affect the phase of the optical mode in the two arms in an anti-parallel fashion. A yet further distinguishing feature compared to Yu related to the optical performance is the use of alternative coupling devices 12 and 13 instead of Y-junction couplers. These types of modulators have been demonstrated with much higher baud rates of 40 GBd, and a capability of extending the baud rate to 80 GBd has also been shown.
It is known that intentional chirp may be generated in a series push-pull LLTWE MZM by displacing selected modulation electrodes away from the optical waveguide such that they produce substantially no phase shifting effect. Walker, U.K. Patent No. GB 2375614. A further known technique of achieving intentional chirp in a LLTWE MZM is by adding a passive capacitive element to adjust the voltage splitting of the incoming modulation signal such that the voltage is split non-equally between the two members of each electrode pair. Walker, U.K. Patent No. GB 2361071.
Other techniques for generating intentional chirp in a MZM, not necessarily related directly to the modulation electrode architecture, are known in the art. In modulators fabricated from lithium niobate (LiNbO3), chirp can be produced by judicious placement of the optical waveguide relative to the fringing electric field. P. Jiang et al., “LiNbO3 Mach-Zehnder Modulators With Fixed Negative Chirp”, IEEE Photon. Tech. Lett., Vol. 8 (10), p. 1319, 1996. U.S. Pat. No. 5,524,076 (Rolland) teaches that chirp can be generated by adjusting the optical splitting elements such that the power is split non-equally between the two MZM arms. It has been shown that a difference in DC bias on the two modulation electrodes in a lumped element InP/InGaAsP MZM with a non-linear phase-voltage characteristic can produce chirp by creating an asymmetry in the optical modulation, especially when used in conjunction with a non-equal optical power split ratio. I. Betty et al., “An empirical model for high yield manufacturing of 10 Gb/s negative chirp InP Mach-Zehnder modulators”, Optical Fiber Communication Conference 2005, Technical Digest, OFC/NFOEC, paper OWE5, 2005. U.S. Pat. No. 6,650,458 (Prosyk) has shown how to generate a continuously adjustable amount of chirp by nesting two parallel MZMs in an external Mach-Zehnder interferometer and diverting a variable amount of power to one or the other MZM.
All of the above techniques for generating intentional chirp fall into three categories: (i) generation of chirp due to an imbalance in the effectiveness of the modulation electrodes (referred to hereafter as “drive imbalance”); (ii) generation of chirp via a non-equal optical power split ratio between the two parallel optical paths (hereafter “power imbalance”); and (iii) generation of chirp by phase modulation separate from amplitude modulation (hereafter “tandem phase modulation”). The three categorical approaches do not produce identical effects in performance, and some of the trade-offs between (i) and (ii) have been quantified in the scientific literature.
It is the object of this invention to provide a means of generating intentional chirp which enables the performance advantages of the LLTWE MZM architecture to be combined with any and all of the above categorical methods of supplying chirp. As a further refinement of the invention, a means of adjusting the degree of power imbalance during operation is provided, therefore allowing the chirp to be tuned, and enabling a dynamic method of changing the chirp proportionality between power imbalance and other chirp generating means.
Although the preferred embodiment of the invention is a MZM with traveling wave electrodes in series push-pull configuration fabricated in InP/InGaAsP, portions of this invention are not constrained to such a MZM architecture, nor is any part of the invention constrained to the InP/InGaAsP material system. It will be obvious to practitioners skilled in the art that the simplicity and adjustability can be advantageously applied to any device where an optical input signal is divided into two or more optical signals and then recombined into one or more output signals, wherein one or more of the signals after division is modulated with an electrical signal. For MZM devices, a range of architectures is also possible. For example, a TWE MZM in parallel push-pull configuration as described by Walker et al. in U.K. Patent No. GB 2384570 could be adapted to use this invention. A Y-junction lumped element MZM with only a single modulation electrode on each arm as per FIG. 1 or FIG. 3 in the Yu patent, could be largely improved through the implementation of the adjustable loss element aspect and/or multiple electrode aspects of this invention. Alternative materials are also possible, including but not limited to LiNbO3, InP/InAlGaAs, GaAs/AlGaAs.
3. Solution to the Problem
None of the prior art references discussed above disclose a TWE MZM where the transmission modulation electrodes in one arm of the waveguide have different shapes or dimensions (e.g., a shorter length or wider width) than those in the other arm of the waveguide. This configuration is capable of a controlled-chirp output signal to meet the needs of a particular telecommunications system. In particular, a negative chirp output signal can be produced.